Ground collapse is a primary excavation risk: it can happen fast and without warning, and a buried worker is likely to suffocate before help comes because soil covers the head or stops the chest expanding. The Code's Figure 8 states that nobody should be inside a trench deeper than 1.5 m until support is in place, lists water seeping into the excavation and tension cracks on the surface among the signs that collapse is near, and notes a wedge of sand over one metre of a two metre deep trench weighs about three tonnes. When selecting methods and controls, consider whether entry can be avoided, the type and strength of the ground (natural or backfilled), moisture, slope, groundwater, faults, nearby watercourses, drains or runoff, work area limits, planned face height, nearby traffic and plant, vibration from other work, adjacent loads such as buildings, tanks, retaining walls and trees, and underground services.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.